The present invention relates to a corona preionization electrode unit for use in gas laser apparatus, e.g. excimer lasers. More particularly, the present invention relates to a corona preionization electrode unit for gas laser apparatus which is capable of generating efficient, stable and uniform corona preionization.
Gas laser apparatus, e.g. excimer lasers, have corona preionization electrodes for irradiating a laser gas with ultraviolet radiation to weakly ionize the laser gas in advance of ionization by the main electric discharge, besides main discharge electrodes for ionizing and exciting the laser gas [for example, see Published Japanese Translation of PCT International Publication No. 8-502145 and Japanese Patent Application Unexamined Publication (KOKAI) No. 10-242553].
There is known a corona preionization electrode structure in which one electrode is axially disposed in a hollow dielectric material and the other electrode, which is a wire-shaped electrode, is wound around the dielectric material to generate a corona discharge, thereby uniformly preionizing the laser gas in the discharge space between the main discharge electrodes [Japanese Patent Application Unexamined Publication (KOKAI) No. 63-227074].
In recent years, however, it has been strongly demanded that excimer laser apparatus for semiconductor exposure and so forth perform a high repetition rate oscillating operation (i.e. it has been demanded that the pulse repetition frequency be increased from 1 kHz, which is the conventional level, to 2 to 3 kHz). Under these circumstances, the above-described electrode structure in which a wire-shaped electrode is wound around a hollow dielectric material suffers from the problem that the wire-shaped electrode undesirably vibrates at such a high pulse repetition frequency.
Meanwhile, a corona preionization electrode structure in which a plate-shaped electrode is placed in contact with a dielectric material is known [Published Japanese Translation of PCT International Publication No. 8-502145]. The known corona preionization electrode structure is shown schematically in FIG. 7.
FIG. 7 is a sectional view in a direction perpendicular to the laser oscillation direction of an excimer laser apparatus. A laser cavity 1 is filled with a laser gas 2. Main discharge electrodes 3 and 4 for exciting the laser gas 2 are disposed to face each other in a direction perpendicular to the laser oscillation direction. The laser gas 2 is circulated by a fan (not shown) so as to form a gas stream between the opposed main discharge electrodes 3 and 4. Corona preionization electrode units 5 are disposed on the upstream and downstream sides, respectively, of the stream of laser gas 2 in parallel to one main discharge electrode 4. The corona preionization electrode units 5 perform a corona discharge operation immediately before the application of a pulse voltage between the main discharge electrodes 3 and 4 to generate a main electric discharge, thereby irradiating the laser gas 2 between the main discharge electrodes 3 and 4 with ultraviolet radiation 6 and thus weakly ionizing the laser gas 2 to promote excitation by the main discharge electrodes 3 and 4. The corona preionization electrode units 5 in Published Japanese Translation of PCT International Publication No. 8-502145 have a structure in which a first electrode 7 is provided to extend through a dielectric tube 8 made of alumina ceramics, and a second electrode 9 formed in a plate-like shape is placed in contact with the outer surface of the tube 8.
As stated above, it has recently been demanded that excimer laser apparatus perform a high repetition rate oscillating operation. It is necessary in order to attain a high repetition rate operation to increase the intensity of corona preionization, prevent the corona light emission intensity from being lowered by the high repetition rate operation and secure the corona preionization electrode structure so that it will not change even in a high-velocity gas stream (because it is necessary to increase the flow velocity of the laser gas in proportion to the pulse repetition frequency).
In the conventional corona preionization electrode structure shown in FIG. 7, the second electrode 9 is a plate-shaped electrode, and this gives rise to problems as stated below.
{circle around (1)} Because the dielectric tube 8 and the second electrode 9 are in plane contact with each other, the field strength is weak. Therefore, the corona preionization intensity cannot always be increased. In addition, because of the plane contact, the condition of contact between the dielectric tube 8 and the second electrode 9 is not always satisfactory, and it is likely that the corona preionization intensity will become nonuniform.
{circle around (2)} Light generated by corona discharge below the contact position of the plate-shaped second electrode 9 is blocked by the second electrode 9 and cannot be used for preionization of the laser gas 2 between the main discharge electrodes 3 and 4. Therefore, the preionization efficiency is unfavorably low.
{circle around (3)} Because the second electrode 9 has a plate-like shape, the stream of laser gas 2 is obstructed by the plate-shaped electrode 9. Consequently, the laser gas 2 stagnates at the surface of the dielectric tube 8, resulting in a reduction in the intensity of light generated by corona discharge.
{circle around (4)} Because of its plate-like shape, the second electrode 9 vibrates as it resists the stream of laser gas 2, causing the corona discharge to become unstable.
In view of the above-described problems with the prior art, an object of the present invention is to provide a corona preionization electrode unit for use in gas laser apparatus, e.g. excimer lasers, which includes a first electrode covered with a dielectric material and a second electrode different from the first electrode, wherein the electric field for corona discharge is concentrated, and light generated by corona discharge is not blocked, and further the laser gas stream is not obstructed, thereby allowing efficient, stable and uniform corona preionization.
To attain the above-described object, the present invention provides a corona preionization electrode unit for use in a gas laser apparatus. The corona preionization electrode unit is disposed in the gas laser apparatus together with a pair of main discharge electrodes for ionizing and exciting a laser gas. The corona preionization electrode unit includes a first electrode covered with a dielectric material and a second electrode placed in contact with the outer surface of the dielectric material around the first electrode. The corona preionization electrode unit is positioned in the vicinity of either one of the main discharge electrodes. The second electrode is a plate-shaped member having a straight edge contacting at least the outer surface of the dielectric material. A portion of the plate-shaped member other than the edge is provided with a plurality of openings for passage of ultraviolet light for preionization and a circulating laser gas.
In this case, it is desirable that the second electrode be in line contact with the outer surface of the dielectric material at the edge thereof, and that a surface of the plate-shaped member in the vicinity of the edge of the second electrode be at an angle to a plane tangent to the outer surface of the dielectric material at the position of the line contact.
In addition, it is desirable that the edge of the second electrode be inserted into a groove formed in the outer surface of the dielectric material to contact the dielectric material.
In addition, it is desirable that the edge of the second electrode contact the outer surface of the dielectric material at a position from which the space between the main discharge electrodes is visible.
In addition, it is desirable that the second electrode be bent or curved in the vicinity of the edge.
The second electrode may be in contact with a plurality of portions of the outer surface of the dielectric material at portions thereof in the vicinity of the edge.
It is desirable that the opening ratio of the openings in the second electrode be not less than 50%.
In the present invention, the second electrode is a plate-shaped member having a straight edge contacting at least the outer surface of the dielectric material around the first electrode, and a portion of the plate-shaped member other than the edge is provided with a plurality of openings for passage of ultraviolet light for preionization and a circulating laser gas. Therefore, the electric field for corona discharge is concentrated, and light generated by corona discharge is not blocked, and further the laser gas stream is not obstructed, thereby allowing efficient, stable and uniform corona preionization. Accordingly, even when the laser apparatus performs an oscillating operation at a high pulse repetition frequency, it is possible to effect preionization of high intensity and high uniformity and hence possible to improve the laser output characteristics to a considerable extent.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.